The known types of transmission systems for which this invention is useful involve the transmission of data frames including at least one synchronisation word and at least one data word. The synchronisation word or words are repetitively transmitted and are ordered within each frame in a selected fashion.
A frame is a set of words where each word is a set of symbols. The symbols, often known as "bits", are used to represent information or data, as well as being used to synchronise the demultiplexer with the received data. Once a frame is transmitted another frame composed of the same number of words each with the same number of symbols as the first frame is transmitted. For example, a frame may consist of two hundred and forty bits split into fifteen words with each word consisting of sixteen bits where one bit is a synchronisation bit and the other fifteen am data bits.
It is known that transmission systems use either interleaved or block synchronisation words. An interleaved synchronisation word is, for example, one where one bit within a word is a synchronisation bit and the other bits of that word are data bits and the interleaved synchronisation word is the time ordered collection of the synchronisation bits of the words of the frame transmitted.
Block synchronisation is where one of the words of the transmitted frame is a synchronisation word, i.e. a word comprising synchronisation bits, the other words of the frame being data words, i.e. words comprising data bits.
These transmission systems can be binary or other multi-valued transmission systems. For the sake of convenience reference will be made to binary transmission systems but it will be understood that the invention is not limited to binary transmission systems. It will be appreciated that the word "bit" in the examples hereafter refers to a symbol having one of two values but would in other multi-value transmission systems refer to a symbol having one of a number of selected values. By binary is meant data values being one of the set {0,1}.
It will be appreciated that a demultiplexer for the mentioned type of transmission system must be synchronised to the data being received. This involves two processes, synchronising to the transmission, and maintaining synchronisation which may also be termed as detecting a lack of synchronisation.
Synchronising to the transmission requires the identification of the start of the data frame.
Maintaining synchronisation or detecting a lack of synchronisation is necessary to ensure that the data received is interpreted correctly.
However, known pdor synchronisation methods generally deal with only one frame of synchronisation data. The result of this is that when the transmission is corrupted due to noise, re-synchronisation often requires the transmission of many frames before synchronisation is achieved. The invention provides a robust and more efficient synchronisation method.
Prior known methods of detecting a lack of synchronisation are vulnerable to short bursts of interference, typified by fading phenomena, resulting in the need to re-synchronise. Whilst re-synchronisation is occurring the reception of intelligible data is interrupted. The invention provides substantial immunity to short bursts of interference.
It will be appreciated that the two processes, establishing a synchronism state and detecting a lack of a synchronism state, may be performed by the same equipment operating in one of two modes. Each mode corresponds to one of the two processes mentioned above. The process of detecting a lack of synchronism state will be discussed first with the process of establishing a synchronism state being discussed thereafter.
A detailed discussion of the above considerations including a theoretical description of the factors to be considered in overcoming the abovementioned problems has been presented by the author at The Institution of Engineers Australia, Communications Conference held in Melbourne, Australia 16-18 October 1990 entitled `A DSP Implementation of a Robust Flexible Receiver/Demultiplexer for Broadcast Data Satellite Communications`.